Data having varying amplitude, such as e.g., ASK (amplitude shift keying) modulated signals, are used in many devices for transmitting communication information from a sender to a receiver. Contactless transponders may communicate with a reader unit using ASK modulation. For ASK modulation, different modulation indexes are defined such as the indexes presented in the ISO 14443 Standard. A first modulation index is provided by type A communication and represents a modulation degree of approximately 100%. A second type of modulation index is represented by type B communication which includes modulation depths of typically 10%.
A type A demodulator may be provided by a comparator unit which has a threshold value close to zero, whereas for the type B demodulator, a filtering operation is required. The filtering operation is introduced because an absolute voltage threshold cannot be defined. Furthermore, in type B modulation systems, a modulated level may change in accordance with the non-modulated level, and an actual modulation index thus may vary between 6% and 30%.
Thus voltage demodulation in a contactless transponder is based on an envelope detector which includes diodes, capacitances and load components. Furthermore, a post-processing circuit may be devised for recovering digital information from the signal envelope. In order to provide demodulation of type A and type B communication signals (i.e., signals having a modulation depth of approximately 100% and signals having a modulation depth of approximately 10%, respectively), two different types of demodulators are provided in the transponder circuit.
It is thus an issue to provide a reliable, accurate and cost-effective circuit arrangement for retrieving communication information contained in varying amplitude data.